This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Cytokine production by the Th1 and Th2 subsets have been associated with susceptiblity to infectious, allergic, and autoimmune diseases. Understanding the molecular events which control lineage-specific cytokine expression would provide useful tools to modulate the Th1/Th2 response. IL-4 production by Th2 cells is regulated by NFAT and the NFAT cofactor, NIP45. We have shown that arginine methylation of NIP45 facilitates its interaction with NFAT and augments IL-4 transcription. Our data positioned the arginine methyltransferase PRMT1 downstream of the T cell receptor, suggesting that arginine methylation may be an important modification in immune receptor signaling pathways. Arginine methylation is countered by the actions of peptidylarginine deiminase 4 (PAD4). NIP45 methylation is negatively regulated by PAD4 via citrullination of arginine residues, which prevents the ability of NIP45 to be methylated by PRMT1, reducing NIP45-induced IL-4 promoter activity. We hypothesize that through actions on NIP45 and other regulatory proteins that PAD4 controls Th cell cytokine expression.